The reduction in memory cell and other circuit size required for high density dynamic random access memories (DRAMs) and other circuitry is a continuing goal in semiconductor fabrication. Implementing electric circuits involves connecting isolated devices through specific electric paths. When fabricating silicon and other material into integrated circuits, it is necessary to isolate devices built into the substrate from one another. Electrical isolation of devices as circuit density increases is a continuing challenge.
One method of isolating devices involves the formation of a semi-recessed or fully recessed oxide in the nonactive (or field) area of the substrate. These regions are typically termed as "field oxide" and are formed by LOCal Oxidation of exposed Silicon, commonly known as LOCOS. One approach in forming such oxide is to cover the active regions with a layer of silicon nitride that prevents oxidation from occurring therebeneath. A thin intervening layer of a sacrificial pad oxide is provided intermediate the silicon substrate and nitride layer to alleviate stress and protect the substrate from damage during subsequent removal of the nitride layer. The unmasked or exposed field regions of the substrate are then subjected to a wet H.sub.2 O oxidation, typically at atmospheric pressure and at temperatures of around 1000.degree. C., for two to four hours. This results in field oxide growth where there is no masking nitride.
However at the edges of the nitride, some oxidant also diffuses laterally. This causes the oxide to grow under and lift the nitride edges. Because the shape of the oxide at the nitride edges is that of a slowly tapering oxide wedge that merges into another previously formed layer of oxide, it has commonly been referred to as a "bird's beak", The bird's beak is a lateral extension or encroachment of the field oxide into the active areas where the devices are formed. Although the length of the bird's beak depends upon a number of parameters, the length is typically from 0.05 micron-0.15 micron per side.
This thinner area of oxide resulting from the bird's beak provides the disadvantage of not providing effective isolation in these regions, and as well unnecessarily consumes precious real estate on the semiconductor wafer. Further, as circuit density commonly referred to as device pitch falls below 1.0 micron, conventional LOCOS techniques fail due to excessive encroachment of the oxide beneath the masking stack. The closeness of the masking block stacks in such instances can result in effective joining of adjacent bird's beaks, thus effectively lifting the masking stacks and resulting in no masking effect to the oxidation.
Also, considerable undesirable stresses are created beneath the nitride at the silicon interface when oxidation occurs. These stresses are in large part due to the formed SiO.sub.2 consuming 2.4 times the volume as elemental silicon it replaces. The pad oxide layer absorbs some of this stress, but additional techniques have been utilized. One such technique used to further alleviate the stress during LOCOS is to use a thin layer of polysilicon which is interposed between the pad oxide layer and the overlying nitride. The goal is to have some of the stress, which would otherwise be subjected upon the silicon substrate, occur in the deposited polysilicon layer. A significant problem, however, is that the polysilicon at the edges of the nitride mask will itself oxidize, forming SiO.sub.2 which is the same composition as the field oxide regions. Such oxidized polysilicon regions are not practically removable, and undesirably result in increased volume of oxide in the bird's beak regions.
The SILO (sealed-interface local oxidation) process is another modification of typical prior art LOCOS processes. In SILO, a thin layer of silicon nitride is formed on the silicon surface. A pad oxide layer (SiO.sub.2) and a nitride film are subsequently deposited. This three layer composite is then photopatterned and etched to overlie desired active area regions and leave desired field oxide regions exposed. The nitride layer is intended to seal an interface between it and the silicon substrate surface to restrict lateral diffusion of oxidants during LOCOS, and thus reduce bird's beak encroachment.
It would be desirable to develop alternate techniques.